Segmented polyurethanes are prepared from hydroxyl terminated low molecular weight polyethers or polyesters or mixtures thereof by reacting these materials with a stoichiometric excess of a diisocyanate to produce an isocyanate terminated prepolymer. This reactive prepolymer is then chain extended with a difunctional compound containing an active hydrogen such as water, glycols, aminoalcohols or diamines.
Diamine-extended polyurethanes have a relatively high urea linkage content due to their diamine extension. This high urea linkage content results in relatively high levels of hydrogen bonding which, in turn, produces strong elastic materials with good flex life properties. However, this high level of hydrogen bonding renders the polyurethane non-thermoplastic. This non-thermoplastic nature severely restricts the processes which may be utilized to fabricate these polyurethanes into useful devices. Techniques such as extrusion, injection molding and heat sealing cannot be utilized due to the polyurethanes' inability to melt and flow prior to decomposition.
Segmented polyurethanes have also been synthesized utilizing relatively low molecular weight diols as chain extension agents. The use of diols produces a segmented polyurethane having reduced levels of hydrogen bonding. As a result of this reduction in hydrogen bonding, these particular polyurethanes have reduced physical properties such as elongation, ultimate tensile strength and flex life as compared to their equivalent diamine extended counterparts. However, this relatively lower level of hydrogen bonding renders the polyurethane thermoplastic and allows it to be extruded, injection molded, heat sealed, etc.
It is known that the mechanical properties and predominately the elastic properties of linear segmented polyurethane block copolymers arise from a two phase microstructure. This two phase structure is derived from the individual chain structure of the polyurethane.
Segmented polyurethane polymer chains may be considered as a series of alternating "soft" and "hard" blocks. Typically, the "soft" blocks are diisocyanate-coupled relatively low melting point polyester or polyether polymers of relatively low molecular weight. The "hard" blocks include single diurethane linkages resulting when a diisocyanate molecule couples two polyester or polyether molecules. More particularly, they are the relatively higher melting urethane or urea chain segments formed by the reaction of diisocyanate with a glycol and/or a diamine chain extender.
The polar nature of the recurring rigid "hard" urethane/urea chain segments results in their strong mutual attraction. This attraction (both inter- and intra-molecular) leads to aggregation and ordering into crystalline and paracrystalline domains (also called pseudo crystalline domains) in a mobile polymer matrix. The high level of urethane and urea hydrogen atoms together with carbonyl and ether oxygen groups permits extensive hydrogen bonding in these systems. The level of this hydrogen bonding restricts the mobility of chain segments to organize extensively into crystalline lattices. The result is a polymer system in which there are at least three levels of association.
Portions of "hard" blocks form discernible crystalline domains while portions of the "soft" blocks form the basis for an amorphous polymer matrix. There is, however, at least a third "phase" which is formed from the complex interaction of paracrystallinity and hydrogen bonding. This leads to the formation of what has been described as "pseudo crosslinks". That is, primary polyurethane chains are crosslinke in effect, but not in fact. The overall consequence is the formation of a labile network of polymer chains which display many of the mechanical, chemical and physical properties of truly crosslinked networks. This type of "pseudo crosslinking" may be partially reversed or enhanced by heat and solvation.
Extrudable, water-extended polytetramethylene ether polyurethane-urea resins are described in Gilding U.S. Pat. No. 4,062,834 et al. The described resins have been rendered extrudable by a reduction and rearrangement of the hydrogen bonds in the polymer chain. This is accomplished by reacting the isocyanate terminated prepolymer with water to form an unstable carbamate which in turn decomposes to form an amine and carbon dioxide. The so formed amine terminated prepolymer is then reacted with another molecule of isocyanate terminated prepolymer. The polyurethanes thus produced prove to be thermoplastic; however, the physical properties such as elongation, ultimate tensile strength and flex life are inferior.
U.S. Patent No. 3,635,907 to Schulze, et al. discloses a polyurethane wherein a mixture of chain extenders such as diamines, hydrazine, or hydrazine derivatives, and aminoalcohols is utilized for chain extension, followed by further addition of a diisocyanate and a diol. The ultimate tensile strength of the polyurethane is varied by varying the quantity of diisocyanate added (column 4, lines 8-11). The reaction between the chain extenders and the NCO-groups of the prepolymer is terminated by the addition of aliphatic alcohols (column 1, lines 70-72). Films produced from polyurethanes derived from linear polyesters admixed with relatively low molecular weight aliphatic diols are said to be non-porous and impermeable to water vapor (column 2, lines 27-28).
The present invention provides segmented polyether polyurethane-urea resins which have superior physical properties such as an increase in strength when wet with water, elongation, ultimate tensile strength, and flex life. Some of the present resins may be readily extruded, injection molded and heat sealed, and can be made into visually clear films that are hydrophilic and are permeable to water vapor. Such films are eminently well suited for the fabrication of wound dressings.